In recent years, software-defined radios that use general-purpose hardware and that can switch between radio communication standards that only correspond to settings on software have been actively studied and developed. Software-defined radios need to deal with radio frequencies from several 10 MHz to several GHz that are generally used.
FIG. 1A shows the structure of a receiver disclosed in Non-Patent Literature 1 as an example of a receiver that receives RF (Radio Frequency) signals. In this receiver, a received RF signal is input through an antenna to an RF circuit composed of band pass filter 160, low noise amplifier (LNA) 161, RF tracking filter 162, and frequency converter 163. Band pass filter 160 eliminates interference signals that lie in an unnecessary bandwidth from the received RF signal so as to prevent the downstream circuits from getting saturated (however, in this case, band pass filter 160 cannot eliminate interference signals having frequencies in the neighborhood of the frequency of the desired signal). The received RF signal that passes through band pass filter 160 is amplified by LNA 161. After RF tracking filter 162 further suppresses the remaining interference signals, frequency converter 163 converts the frequency of the received RF signal using a clock signal generated by clock generator 164, and then a baseband section performs signal processes such as filtering for the resultant signal.
From a point of view of cost and the size of circuit area for software-defined radios, it is not preferred that components that differ in characteristics be implemented and switched between applicable radio communication standards. In particular, reducing of the number of band pass filters that are integrated in a chip is difficult and has become a critical technical issue in order to accomplish software-defined radios. To reduce the number of band pass filters, a technique that allows the pass bandwidth of a band pass filter to become variable or another technique that allows signals having frequencies of several 10 MHz to several GHz to pass might be contemplated. On the other hand, band pass filters located upstream of the LNA need to satisfy both high linearity and low noise characteristics. Although passive filters such as surface acoustic filters (SAWs) excellently satisfy such characteristics, it is difficult to adjust the pass bandwidth of passive filters.
Thus, in receivers applicable for software-defined radios, SAW filters that have wide pass bandwidths might be a hopeful candidate for band pass filters. However, in this case, depending on the frequency of a desired signal, interference signals having frequencies up to 10 times higher than the frequency of the desired signal can be input to the LNA and the frequency converter. Ideally, a frequency converter uses a mixer that multiplies the received RF signal by the LO signal and outputs signals of which an addition and a subtraction for the frequency of the received RF signal and the frequency of the LO signal are performed. However, actually, because of harmonics of the LO signal and nonlinearity of the mixer, the frequency converter also converts the frequencies of interference signals other than the desired signal. In particular, if the LO frequency of the LO signal is low, it is difficult to transfer the LO signal that is a sine wave. Rather, it is preferable that the LO signal that is a square wave be transferred from the point of view of the size of circuit area and power consumption. However, since an LO signal that is a square wave contains many odd-order harmonics, crosstalk with the received RF signal has become a critical problem (even-order harmonics can be eliminated using a differential circuit structure).
The receiver disclosed in Non-Patent Literature 1 uses both a mixer (FIG. 1B) called harmonics eliminating mixer located in frequency converter 163 and RF tracking filter 162 in order to solve the foregoing problem. The harmonics eliminating mixer uses a three-phase square LO signal having phases that vary by 45 degrees each. For example, base band I signal having a phase of 0 degree is obtained by multiplying the received RF signal by an LO signal having phases of −45 degrees, 0 degree, and 45 degrees, weighting the results with gains of 1, √2, and 1, respectively, and adding the results. Base band signal having a phase of 90 degrees can be obtained by multiplying the received RF signal by an LO signal having phases of 45 degrees, 90 degrees, and 135 degrees, respectively, weighting them with the foregoing gains, and adding the results. Likewise, inverted signals of base band I signal and base band Q signal can be obtained by using an LO signal having phases of 135 degrees, 180 degrees, and 225 degrees and an LO signal having phases of 225 degrees, 270 degrees, and 315 degrees, respectively. In other words, to demodulate base band I signal and base band Q signal, an LO signal having a total of eight phases that vary by 45 degrees each is used. By weighting an LO signal having phases that vary by 45 degrees each and adding the results, the frequency conversion using an LO signal that approximates a pseudo-sine wave, although it is actually a square LO single, can be performed (FIG. 2). Since the third-order and fifth-order harmonics of the artificial sine wave are 0, crosstalk between interference signals having these frequencies and the received signal can be eliminated.
As another example as disclosed in Non-Patent Literature 2, FIG. 3 shows the structure of a receiver that does not use an RF tracking filter. In this receiver, mixer 182 uses an LO signal having a total of eight phases that vary by 45 degrees each so as to prevent crosstalk between interference signals of the third-order and fifth-order harmonics and the received signal like Non-Patent Literature 1. The receiver disclosed in Non-Patent Literature 2 is different from the receiver disclosed in Non-Patent Literature 1 in that instead of an RF tracking filter, two types of band pass filters 180 and 184 are used and channel pass filter 188 switches from one band selection filter to another corresponding to the frequency of a desired signal. The receiver disclosed in Non-Patent Literature 2 has LNA 181, mixer 182, and 8-phase clock generator 183 that corresponds to band pass filter 180; and LNA 185, mixer 186, and 4-phase clock generator 187 that corresponds to band pass filter 184. For example, in the case that the bandwidth of band pass filter 180 ranges from 0.4˜2.5 GHz and the bandwidth of band pass filter 184 ranges from 2.5˜6 GHz, if the frequency of the desired signal is 0.4 GHz, band pass filter 180 is used. Since the frequency seven times higher than 0.4 GHz is 2.8 GHz, an interference signal of the seven-order harmonic of an LO signal can be eliminated by band pass filter 180.